Backlights for liquid crystal displays (LCDs) are sometimes formed using a rectangular plastic waveguide (or lightguide) with one or more LEDs optically coupled to an edge of the waveguide. The LEDs may include a phosphor coating to create white light.
FIG. 1 is a top down view of a portion of a backlight waveguide 10 with three identical LEDs 12 optically coupled to an edge of the waveguide 10. Each LED may comprise a blue emission LED die 14 (e.g., a GaN LED) mounted on a submount 16, a phosphor layer (not shown) over the die to contribute red and green light components to create white light, and a domed lens 18. The lens 18 is typically hemispherical so that the light emission is Lambertian. The lens is formed of a high index of refraction (n) plastic or silicone to increase the light extraction from the LED die 14 by increasing the critical angle at the die interface. Therefore, by using such a lens 18, the total internal reflection (TIR) within the die 14 is reduced compared to if the LED die 14 had no lens and had a die/air interface.
In a backlight waveguide illuminated by multiple LEDs along its edge, the LED light needs to merge and mix within the waveguide so as to be generally uniform. This mixing naturally occurs as the light from each LED spreads out within the waveguide and merges. However, the light near the edge of the waveguide 10, in the mixing region 20, is not uniform, so that part of the waveguide 10 near the LEDs 12 is not used to backlight an LCD 24 (FIG. 2). Light rays (shown as lines with arrows) are refracted toward the normal as light goes from a lower n to a higher n, such as from the air gaps of FIG. 1 into the plastic (e.g., PMMA) waveguide 10. This refraction increases the depth of the mixing region 20 near the edge. This mixing region adds width to the backlight, which is undesirable. One solution to shorten the mixing region is to decrease the pitch of the LEDs, but this adds cost.
FIG. 2 illustrates a side view of the backlight of FIG. 1 showing how a light ray 25 from an LED 12 striking the top surface of the waveguide 10 at greater than the critical angle is totally internally reflected by the smooth top surface of the waveguide 10. Such TIR is important to prevent non-uniform leaking of light through the top surface. The waveguide 10 typically has prisms 22 or roughening on its bottom surface to reflect light upward to uniformly leak out the top surface to illuminate an LCD 24.
The waveguide 10 should be thick enough to receive a high percentage of the emitted LED light coupled to its edge. A reflector around the LEDs may be used to direct side light from the LEDs toward the waveguide edge, but such a reflector adds space and cost.
What is needed is optics for LEDs that optically couple an LED die to a backlight so that the mixing region is shorter and so that the waveguide can be made thinner without losing efficiency.